Eryngium planum (E. armatum, E. caeruleum, E. intermedium, E. latifolium, E. planifolium) | Flat Sea Holly ‘Jade Frost’
Sea Holly has been grown in American gardens since the 1800’s. Its an easy-to-grow fancy thistle with beautiful metallic, almost iridescent, bright blue, thistle-like flowers that sit atop 3 ft tall plants. The ‘Jade Frost’ varieties blooms are cream when they first appear, then turn blue, creating bicolor blooms on one plant. As they age, the spiny narrow bracts turn blue. The blue color of the flower will intensify in higher temperatures. What makes ‘Jade Frost’ Sea Holly so special though is the low-growing blue-green foliage that has a stunning creamy white margin along the edges. When night temps are still cool, the young leaves may even have a pink-tinge to them. Deer- and rabbit-resistant, sea holly’s foliage looks great even before the flowers take off.
Sea Holly plants love sunshine and normal or sandy soil, and once established, they are extremely drought resistant and require little care. They make great specimens for low-water gardens. Seeds can be started directly outside by scattering outside in late autumn or can be started indoors, but may require vernalization/ stratification. Move seedlings to their permanent location when they are still small because Eryngium have large taproots and won’t transplant well once mature.
The species is hermaphrodite (has both male and female organs) and is self fertile. Eryngium plants may or may not flower the first year from seed, but should come back with a vengeance in following years. They are valued by many for their prolonged bloom time. If you deadhead them they will keep blooming well into fall. Seeds will self-sow, but not to the point of becoming invasive.
Attracts bees, flies, butterflies, beetles, and other pollinators like crazy! This was our garden-winner for ‘most visited’ by pollinators as well as having the most diverse group of pollinators too! Plus are deer and rabbit resistant.
If starting seeds indoors, Blue Sea Holly seeds will require a 60 day period of cold-moist stratification before the seeds will germinate. Or if you plan on starting them outdoors you can sow the seeds in the fall and let nature do its thing.
Cold stratification is a term for a procedure that can improve certain seed germination rates by 300-400%. Cold stratification is designed to mimic the winter season’s cold and moist weather that unlocks a seeds protective germination mechanisms and triggers the seed to sprout out of its dormant state. (Most perennial plant seeds (such as native wildflowers) require this combo of cold and damp to germinate.) In nature, this occurs naturally, but by doing the process yourself in a controlled environment, you keep the young seeds safe from any animals that might eat them and they’ll also be less likely to succumb to rot or mildew, leaving more seeds to grow.
There are many methods to cold-stratify seeds, but the two key factors are always moisture and cold. The best and easiest way is to put a small amount of moist/wet sand/soil in a zip baggie and store in the refrigerator for 2 months time (or more). Once stratified, surface sow the seeds (and sand) and gently press into the dirt. Germination will occur with heat humidity and light indoors, or if outdoor, once spring temps and sunlight are adequate. Be sure to plant stratified seeds within a day or so after removing them from the refrigerator because as soon as they warm up they will be ready to grow and may start to sprout. Move seedlings to their permanent location when they are still small because Eryngium have large taproots and resent being transplanted once mature.
Almost all Eryngium make excellent, long lasting cut flowers. Harvest them when the entire flower heads and bracts turn blue so the flowers will keep their beautiful blue color after drying.
Sun exposure: Full sun
Mature height: 3 ft
Mature width: 2 ft
Hardiness zones: 4- 9